Check-actuated control mechanisms – Including means to test validity of check – By testing material composition
Patent
1998-02-02
1999-08-03
Bartuska, F. J.
Check-actuated control mechanisms
Including means to test validity of check
By testing material composition
G07D 508
Patent
active
059312777
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a validation apparatus and method for articles of value; particularly, but not exclusively, coins and banknotes.
BACKGROUND OF THE INVENTION
Such validators generally comprise a sensing system for generating two or more measurement signals, and a processing system for determining acceptability based on the signals and on stored data defining acceptability criteria corresponding to a valid type of article.
The acceptability criteria generally define an area or volume (in a measurement space defined by axes corresponding to the measurement signals) determined by, and encompassing, the statistical distribution of measurements from a population of known genuine articles.
The distributions of genuine articles may overlap with those of others, or with those of forgeries, counterfeits or slugs.
GB-A-2272319 discloses a coin validator using an acceptance region with a curved boundary.
EP-A-0367921, EP-A-0505609, U.S. Pat. No. 5,351,798 and WO-A-92/18951 disclose coin validators using acceptance regions having an ellipsoidal or circular boundary.
WO-A-92/18951, GB-A-2251111 and U.S. Pat. No. 5,351,798 disclose coin validators in which a coin is classified as one of several types in dependence upon the Mahalanobis distance (i.e. the square of the Euclidean distance in a space in which the measurements are each normalised by the variance) from the coin measurement to the center of the distribution of each type.
EP-A-0560023 discloses a banknote validator in which a banknote is accepted as genuine if its measurements define a point within a predetermined Mahalanobis distance from the center of a valid banknote distribution.
An acceptance region boundary defined by a fixed Mahalanobis distance corresponds to an ellipsoidal boundary, and also defines a contour of equal probability (assuming the distribution of genuine coins is unimodal and Normal (Gaussian)) that measurements of a genuine coin are likely to fall within the boundary.
The above-described systems may represent an advance in many areas of validation. Their operation is, however, predicated on the unspoken assumption that the probability of given measurements being associated with a particular item type (the a posteriori probability) is well correlated with the probability of an item of that type exhibiting those measurements.
SUMMARY OF THE INVENTION
We have realised, however, that under some circumstances this assumption is inaccurate. The present invention, in one aspect, is concerned with validation which takes into account factors relevant to the a priori probability to calculate the a posteriori probability rather than relying solely on measurement distributions.
In one aspect, the present invention takes into account the relative frequencies of the occurrence of different types of coins and counterfeit coins or slugs in determining the acceptance region boundary. Thus, in accordance with the invention, a coin is identified as being of a particular type not only in dependence upon the distribution of measurements from coins of that type but also in dependence on the relative frequency of occurrence of coins of that type, relative to other types.
Preferably, this leads to an acceptance boundary at a point where the relative measurement distribution probabilities of the coin being of first or second types are in a predetermined, non unity, ratio.
This therefore equates to the Mahalanobis distances to the center of the distributions of two coin types (for example a valid coin and a slug) being in a relationship corresponding to equal sums of the respective Mahalanobis distances with respective predetermined constants at least one of which is non zero.
The invention may be practised by calculating Mahalanobis distances for the first and second coin distributions and then adjusting one or both of the distances, but this is not essential.
Thus, this aspect of the invention enables an increased accuracy in discriminating particular known slug types by taking into account their probability of occurrence.
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Allan Richard Douglas
Campbell Bernard Joseph
Hutchinson Derek
Mars Frank
Bartuska F. J.
Jaketic Bryan J.
Mars Incorporated
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